Bonding structure with buffer layer and method of forming the same

ABSTRACT

A bonding structure with a buffer layer, and a method of forming the same are provided. The bonding structure comprises a first substrate with metal pads thereon, a protection layer covered on the surface of the substrate, a first adhesive metal layer formed on the metal pads, a buffer layer coated on the protection layer and the metal pads, a first metal layer covered on the buffer layer, and a second substrate with electrodes and a bonding layer thereon. The first metal layer, the electrodes and the bonding layer are bonded to form the bonding structure. Direct bonding can be performed through surface activation or heat pressure. The method uses fewer steps and is more reliable. The temperature required for bonding the structure is lower. The bonding density between the contacted surfaces is increased to a fine pitch. The quality at the bonding points is increased because fewer contaminations between the contacted surfaces are generated.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 10/829,060, filed Apr.20, 2004, now U.S. Pat. No. 7,183,494, issued Feb. 27, 2007, which isincorporated herewith by reference.

FIELD OF THE INVENTION

The present invention generally relates to a bonding structure forbonding semiconductor material or metal surface to substrate, and amethod of forming the same, and more specifically to a bonding structurewith a buffer layer, and a method of forming the same. The bondingstructure can be applied to bond integrated circuit (IC) or chip tosubstrate.

BACKGROUND OF THE INVENTION

There are a variety of conventional bonding techniques for bondingsemiconductor material or metal surface to substrate. One of them is theanisotropic conductive film (ACF) bonding technique. The ACF bondingtechnique places a layer of anisotropic conductive film containingconductive particles between chip and the device to be bonded, and bondsthe chip and the device together by melting the anisotropic conductivefilm using heat and pressure. It also forms conductive channel by usingmetal pads, metal bumps and conductive particles. The disadvantage ofthis technique is that it can not meet finer pitch requirement. For afiner pitch between the metal pads and the metal bumps, conductiveparticles will flow because of heat and pressure being applied. Therebytwo adjacent conductive points may be short. Thus, the technique can notmeet finer pitch requirement. The bonding density of this ACF bondingtechnique can only reach to as small as 50 um pitch.

Another conventional bonding technique is using non-conductive film(NCF) for bonding. The difference between the NCF bonding technique andthe ACF bonding technique is that the former does not contain anyconductive particle in the adhesive material. The bonding structureusing this NCF bonding technique uses heat and pressure to melt thenon-conductive film. After the non-conductive film has consolidated, thegenerated contractive stress bonds the chip and the device together.Although the bonding density is high for this NCF bonding technique, thebonding of the chip and the device is maintained only by mechanicalforce. That is, the contractive stress generated by the film has tomaintain the conducting quality of the contact points. Once the filmbears too much stress, the contact surface among the film, integratedcircuit and substrate will produce lamination and increase theresistance after bonding.

Another method is Au—Au diffusion bonding technique. Because its bondingtemperature is too high and metal oxides will be formed on the surfacesof metal layers, covalent bonds will limit free electrons of metal.Therefore, it is hard to form metal bonds between two bonding surfaces.Also, the electrical conductivity comes from tunnel-through effect thatgenerates higher contact impedance. Therefore, it is not suitable tofine pitch applications too.

U.S. Pat. Nos. 5,407,506 and 5,427,638 disclose respectively surfaceactivation methods. The surface activation methods disclosed in theseU.S. patents mainly bombard the polished surfaces and the cleanedsurfaces by oxygen ions, fluorine ions, or their mixture to activatethese surfaces. Then particles on these activated surfaces are removedand these activated surfaces are contacted under room temperature tocomplete the activation bonding.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above-mentioneddrawback of conventional bonding methods that need high bondingtemperature and to increase the bonding density and the quality ofbonding surfaces. The primary object of the present invention is toprovide a bonding structure with a buffer layer, and a method of formingthe same. The bonding method of the invention combines Au—Au diffusionbonding technology to perform direct bonding with surfaces. Using such abuffer layer, it creates a new bonding structure and its process usesfewer steps and is more reliable.

The bonding structure of the invention comprises mainly a firstsubstrate with metal pads thereon, a protection layer, a first adhesivemetal layer, a buffer layer, a first metal layer and a second substrate.The protection layer is covered on a surface of the first substrate, thefirst adhesive metal layer is formed on the metal pads, the buffer layeris coated on the protection layer and the metal pads, and the firstmetal layer is covered on the buffer layer. A surface of the secondsubstrate has independently distributed electrodes and a bonding layerthereon. The first metal layer, the electrodes and the bonding layer arebonded together to complete the bonding structure.

There are two preferred embodiments of the bonding structure accordingto the present invention. In the first preferred embodiment, the bufferlayer coated on metal pads and the buffer layer coated on the protectionlayer are independently distributed. In the second preferred embodiment,the buffer layer coated on metal pads and the buffer layer coated on theprotection layer are connected. In both preferred embodiments, theelectric connection between the first adhesive metal layer and the firstmetal layer can have various structures to implement.

Using the bonding structure of the present invention, the bondingprocess uses fewer steps than the conventional surface activationmethod. The bonding process of the invention mainly comprises two steps.First, a bonding structure with a buffer layer is formed. Then thesurface of the first metal layer, the bonding layer on the secondsubstrate and the surface of metal pads are directly bonded together.Because the bonding process uses fewer steps, it is more reliable. Aftercompleting the bonding structure, under-fill can be applied to increasethe reliability of the bonding structure.

The direct bonding can be performed through surface activation, surfaceactivation plus heat pressure, or heat pressure only.

The bonding method of the invention requires lower bonding temperature.Therefore, it resolves the drawback of conventional bonding methods thatneed high bonding temperature. The bonding density between the contactedsurfaces is increased to a fine pitch. The quality at the bonding pointsis increased because fewer contaminations between the contacted surfacesare generated. The bonding structure and the bonding method of theinvention can be applied to bond integrated circuit or chip tosubstrate.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become better understood from a careful readingof a detailed description provided herein below with appropriatereference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be understood in more detail by reading thesubsequent detailed description in conjunction with the examples andreferences made to the accompanying drawings, wherein:

FIG. 1 shows a cross-sectional view of the bonding structure with abuffer layer before bonding according to the invention;

FIG. 2 shows a top view of the structure shown in FIG. 1;

FIGS. 3 a and 3 b show respectively cross-sectional views of the bondingstructure without and with under-fill after bonding the structures shownin the upper and the lower figures of FIG. 1;

FIG. 4 illustrates another cross-sectional view of the bonding structurewith a buffer layer before bonding according to the invention;

FIG. 5 shows a top view of the structure illustrated in FIG. 4;

FIGS. 6 a and 6 b show respectively cross-sectional views of the bondingstructure without and with under-fill after bonding the structures shownin the upper and the lower figures of FIG. 3;

FIGS. 7 a˜7 c show A-B cross-sectional views of FIG. 2 to illustratethree different kinds of structures for the electric connection betweenthe first adhesive metal layer and the first metal layer in the upperfigure of FIG. 1;

FIGS. 8 a˜8 c show the modified structures of electric connectioncorresponding to FIGS. 7 a˜7 c;

FIGS. 9 a˜9 c show A-B cross-sectional views of FIG. 5 to illustratethree different kinds of structures for the electric connection betweenthe first adhesive metal layer and the first metal layer in the upperfigure of FIG. 4;

FIGS. 10 a˜10 c show the modified structures of electric connectioncorresponding to FIGS. 9 a˜9 c;

FIGS. 11 a and 11 b show two different embodiments of covering a secondmetal layer on the top of electrodes and the bonding layer above thesecond substrate;

FIG. 12 shows the bonding process of the present invention; and

FIG. 13 shows curves of experimental results of direct bonding with andwithout a buffer layer, the horizontal axis shows the bondingtemperature while the vertical axis shows the bonding strength.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross-sectional view of the bonding structure with abuffer layer before bonding according to the invention. In thisembodiment, there are metal pads 103 a and 103 b on a surface of thefirst substrate 101 shown in the upper figure of FIG. 1. A protectionlayer 105 is covered on that surface of the first substrate 101. A firstadhesive metal layer 110 is formed on the metal pads 103 a and 103 b.Buffer layers 107, 107 a and 107 b are respectively coated on theprotection layer 105 and the first adhesive metal layer 110 over themetal pads 103 a and 103 b. A first metal layer 109 covers buffer layers107, 107 a and 107 b.

Metal pads 103 a and 103 b are used as conducting circuit for the firstsubstrate 101. They can be made of aluminum (Al) or copper (Cu). Theprotection layer 105 protects the integrated circuit on the firstsubstrate 101. The first substrate can be a silicon (Si) substrate. Thematerial for the buffer layer can be polyimide. The material for thefirst metal layer can be chosen from one of gold (Au), aluminum orcopper. It is worthy to mention that the buffer layer coated on thefirst adhesive metal layer over the metal pads and the buffer layer onthe protection layer are independently distributed in the embodiment.

The lower figure of FIG. 1 illustrates the elements bonding to the upperfigure. As shown in the lower figure, the bonding elements include asecond substrate 111, electrodes 113 a and 113 b on the second substrate111, and a bonding layer 115. The bonding layer 115 and electrodes 113 aand 113 b are independently distributed on the second substrate 111.

Electrodes 113 a and 113 b on the second substrate 111 are alignedrespectively to metal pads 103 a and 103 b on the surface of the firstsubstrate 101. The second substrate 111 is generally a glass substrateor a substrate made of polymer or ceramic.

FIG. 2 shows a top view of the structure shown in FIG. 1. FIG. 1 is anA-C cross-sectional view of FIG. 2.

Referring to the upper and the lower figures of FIG. 1, the first metallayer 109, electrodes 113 a and 113 b and the bonding layer 115 arebonded directly to complete the bonding structure. FIGS. 3 a and 3 bshow respectively cross-sectional views of the bonding structure withoutand with under-fill after bonding the structures shown in the upper andthe lower figures of FIG. 1. Label 301 in FIG. 3 b shows the region ofunder-fill.

FIG. 4 illustrates another cross-sectional view of the bonding structurewith a buffer layer before bonding according to the invention. Thedifference between this embodiment and the embodiment illustrated inFIG. 1 is that the buffer layer coated on the first adhesive metal layerover the metal pads and the buffer layer on the protection layer areconnected, as shown in label 401 of FIG. 4.

FIG. 5 shows a top view of the structure illustrated in FIG. 4. FIG. 4is an A-C cross-sectional view of FIG. 5.

Referring to the upper and the lower figures of FIG. 4, the first metallayer 109 is bonded directly to electrodes 113 a and 113 b and thebonding layer 115. FIGS. 6 a and 6 b show respectively cross-sectionalviews of the bonding structure without and with under-fill after bondingthe structures shown in the upper and the lower figures of FIG. 3. Label601 in FIG. 6 b shows the under-fill region.

To increase the reliability of the bonding structure, under-fill can befurther applied after bonding.

For the structure illustrated in the upper figure of FIG. 1 and FIG. 4,the electric connection between the first adhesive metal layer and thefirst metal layer can have various structures to implement. FIGS. 7 a˜7c show A-B cross-sectional views of FIG. 2 to illustrate three differentkind of structures for the electric connection between the firstadhesive metal layer and the first metal layer shown in the upper figureof FIG. 1.

Referring to FIG. 7 a, the buffer layer on the first adhesive metallayer 110 and the metal pad 103 b is separated, as shown in labels 707 aand 707 b. The metal layer 709 a covers directly on the separated bufferlayer 707 a and 707 b, as well as on the first adhesive metal layer 110located between buffer layer 707 a and 707 b. Referring to FIG. 7 b, themetal layer 709 b covers directly on and around the buffer layer 707,and contacts with the first adhesive metal layer 110. The size of thecontact area depends on the design. Referring to FIG. 7 c, anelectroplating metal is electroplated to fill the hollow, as shown inlabel 708, made by the separated buffer layer 705. Then a metal layer709 c is sputtered on the electroplating metal 708 and the buffer layer705. This forms electric conduct among the metal layer 709 c, theelectroplating metal 708 and the first adhesive metal layer 110. Theelectroplating metal 708 and the metal layer 709 c can use two differentmetals or the same metal.

The structure of electric connection shown in each of FIGS. 7 a˜7 c canalso have different variations by adding an adhesive layer between themetal layer and the buffer layer. FIGS. 8 a˜8 c show the modifiedstructures of electric connection corresponding to FIGS. 7 a-7 c. Labels806 a˜806 c are respectively their corresponding added adhesive layers.

Similarly, for the structure in the upper figure of FIG. 4, the electricconnection between the first adhesive metal layer and the first metallayer can be implemented as three different kinds of structures shown inFIGS. 7 a˜7 c. Also, it can further be designed to have the samemodification as shown in FIGS. 8 a˜8 c. FIGS. 9 a˜9 c show A-Bcross-sectional views of FIG. 5 to illustrate three different kinds ofstructures for the electric connection between the first adhesive metallayer and the first metal layer shown in the upper figure of FIG. 4. Thesame description is not provided herein. FIGS. 10 a˜10 c show themodified structures of electric connection corresponding to FIGS. 9 a˜9c. The same description is not provided herein either.

According to this invention, the top of electrodes 113 a and 113 b andthe bonding layer 115 above the second substrate 111 can further becovered by a second metal layer in the structure of the lower figure ofFIG. 1 and FIG. 4. FIGS. 11 a and 11 b show two different embodiments.In FIG. 11 a, a second metal layer 1101 is covered on the bonding layer115. In FIG. 11 b, a second metal layer 1103 is covered on electrodes113 a and 113 b as well as the bonding layer 115.

In the structure before bonding, when the outmost layers of the firstsubstrate and the second substrate are metal layers, covalent bonds willlimit free electrons of metal because of the metal oxide formed on thesurfaces of metal layers. Therefore, it is hard to form metal bondsbetween two bonding surfaces. Also, the electrical conductivity comesfrom tunnel-through effect that generates higher contact impedance.Therefore, direct bonding of the present invention can be performedthrough surface activation. It removes oxide contaminants on thesurfaces of metal layers, i.e., removes covalent bonds on the surfaces,and controls the junction gap and the energy distribution bycompensating flatness of bonding surfaces. Then it can form metal bondsbetween two bonding surfaces and accomplishes the bonding.

Using the bonding structure of the present invention, the bondingprocess uses fewer steps and is more reliable than the conventionalsurface activation method. FIG. 12 shows the bonding process of thepresent invention. As shown in block 1201, a bonding structure with abuffer layer of the invention is formed first. It includes providing afirst substrate with metal pads thereon, covering a protection layer,forming a first adhesive metal layer, coating a buffer layer, covering afirst metal layer, and providing a second substrate with electrodes andan independently distributed bonding layer thereon. Then the surface ofthe first metal layer, the bonding layer on the second substrate and thesurface of metal pads are bonded together, as shown in block 1203.

As mentioned before, the bonding method may use direct bonding or addsfurther under-fill to increase reliability. Direct bonding can beperformed through surface activation, surface activation plus heatpressure, or heat pressure only. Surface activation can be done bybombarding using physical property of plasma, exposing to ultravioletray, or cleaning with chemical compound to remove particles and oxidecontaminants on the surfaces of first metal layer, bonding layer andelectrodes. After bonding, the bonding junction will vanish. Therefore,this perfect surface activation bonding process has excellent electriccharacteristic.

The bonding method of the invention uses mainly the innovative bondingstructure of the invention, i.e., using a buffer layer and combiningAu—Au diffusion bonding technology to perform direct bonding withsurfaces. The method uses fewer steps and is more reliable. Afterbonding, the bonding junction will vanish. Therefore, this perfectsurface activation bonding process has excellent electriccharacteristic.

FIG. 13 shows curves of experimental results of direct bonding with andwithout a buffer layer. The horizontal axis shows the bondingtemperature in unit ° C. while the vertical axis shows the bondingstrength in unit Kg. The two curves indicated by the structure 2, whichis the structure with a buffer layer, respectively illustrate bondingresults by directly applying 40 Kgf-13.3 Mpa heat pressure afterperforming surface activation and without performing surface activation.On the other hand, the two curves indicated by the structure 1, which isthe structure without a buffer layer, respectively illustrate bondingresults by directly applying 40 Kgf-13.3 Mpa heat pressure afterperforming surface activation and without performing surface activation.

With analysis to the two curves without performing surface activation,when the bonding temperature is greater than 200° C., the bondingstrength becomes stronger as the bonding temperature increases. Also,the bonding strength of the structure 2 is stronger than that of thestructure 1. When the bonding temperature is below 150° C., withanalysis to the two curves with performing surface activation, thebonding strength becomes stronger as the bonding temperature increases.Also, the bonding strength of the structure 2 is stronger than that ofthe structure 1. Therefore, the bonding structure with a buffer layer ofthe present invention has stronger bonding strength even at lowtemperature. It can perform surface activation or direct bonding withheat pressure.

Although the present invention has been described with reference to thepreferred embodiments, it will be understood that the invention is notlimited to the details described thereof. Various substitutions andmodifications have been suggested in the foregoing description, andothers will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the invention as defined in the appended claims.

1. A bonding method for a bonding structure with a buffer layer,comprising the steps of: preparing a first substrate; forming aplurality of metal pads on a surface of said first substrate; coveringsaid surface of said first substrate with a protection layer; forming afirst adhesive metal layer on said plurality of metal pads; coating abuffer layer on said protection layer and said first adhesive metallayer; covering said buffer layer with a first metal layer, said firstmetal layer being independently distributed on surface areas of saidbuffer layer opposite to said protection layer and said first adhesivemetal layer; preparing a second substrate; forming a plurality ofelectrodes on said second substrate; forming a bonding layer on saidsecond substrate, said bonding layer and said plurality of electrodesbeing independently distributed on said second substrate; and bondingsaid first metal layer, said plurality of electrodes and said bondinglayer to complete said bonding structure.
 2. The bonding method asclaimed in claim 1, wherein said first metal layer, said plurality ofelectrodes and said bonding layer are bonded by direct bonding.
 3. Thebonding method as claimed in claim 2, wherein said direct bonding isperformed through surface activation.
 4. The bonding method as claimedin claim 3, wherein said surface activation is done by bombarding usingphysical property of plasma to remove particles and oxide contaminantson the surfaces of said first metal layer, said bonding layer and saidplurality of electrodes.
 5. The bonding method as claimed in claim 3,wherein said surface activation is done by exposing to ultraviolet rayto remove particles and oxide contaminants on the surfaces of said firstmetal layer, said bonding layer and said plurality of electrodes.
 6. Thebonding method as claimed in claim 3, wherein said surface activation isdone by cleaning with chemical compound to remove particles and oxidecontaminants on the surfaces of said first metal layer, said bondinglayer and said plurality of electrodes.
 7. The bonding method as claimedin claim 2, wherein said direct bonding is performed through surfaceactivation plus heat pressure.
 8. The bonding method as claimed in claim2, wherein said direct bonding is performed through heat pressuredirectly.
 9. The bonding method as claimed in claim 1, whereinunder-fill is further applied between said first substrate and saidsecond substrate to increase the reliability of said bonding structure.